What exactly do you want to learn. Is it CRM-Service, CRM-Sales, CRM-Marketing, CRM middleware, SAP Mobile, or Mobility for SAP. Do you want to be a functional or technical consultant, architect or all-rounder?.

Depending on your answer(s) depends on what books you want. Either way, take a look at sap-press.com, although sometimes the books are cheaper on Amazon and other book providers.